A fluid can be defined as a substance that undergoes continuous deformation, when subjected to a shear stress. The resistance offered by a real fluid to such deformation is called its viscosity. All fluids have a viscosity and this property causes friction (1). The viscosity of a Newtonian fluid is constant, if static pressure and temperature are fixed. Rheological data are indispensable in the design of pumps, compressors, heat exchangers, packed columns, fluidized beds, distillation columns, pipelines, decanters, and many other pieces of equipment. In addition, rheological properties are utilized in the quality control of many consumer products. Certain materials are more acceptable to consumers if they have specific rheological properties (2).
In order to fully determine the viscosity values of a fluid, careful experimentation is required. Temperature and pressure affect viscosity of a fluid. Effects of pressure on viscosity of a fluid are negligible at pressures less than 40 atmospheres, and the viscosity of fluids is not significantly affected by relatively low pressure (3). Viscosity of Newtonian fluids is determined by measuring the shear stress at one known shear rate, whereas that of all non-Newtonian fluids is determined at more than one shear rate.
There are two main groups of viscometers that may be used to determine the viscosity of a fluid. One type of a viscometer is designed so that a fluid flows through a tube, channel or orifice, while the other type of a viscometer is constructed so that a fluid is sheared between moving surfaces (4). Several forms of viscometers have been constructed for the determination of viscosity from knowledge of the pressure drop-flow rate relationship for a tube. Such a flow may be achieved simply as a result of gravitational head. This type of viscometer is not very suitable for measurements with non-Newtonian fluids, not only because of the non-uniform shear stress across the tube section, but also because the rate of flow through the tube varies with time as the level falls. To overcome these shortcomings, a constant velocity of a fluid through a tube is maintained by pressure from a gas reservoir. For this type of a viscometer, ratios of the length to the diameter of a tube normally exceed 50 to eliminate the end effects of the tube (4). Although a few capillary tube viscometers (sometimes called extrusion rheometers) are commercially available, it is common practice to construct them in the laboratory. In constructing such viscometers, several capillary tubes with a range of lengths and diameters are needed. Tubes with internal diameters from 1/32 inch to ¼ inch are typically used (5).
James et al. (6) reported a consistent set of high precision viscosity data for water at various temperatures and for 20, 30 and 40% (by weight) sucrose solutions. These were obtained using a glass capillary viscometer with an extensively flared capillary to neglect the kinetic energy correction term for this viscometer. These data are recommended as reliable standards for capillary viscometer calibration. Sirivat (7) described the results of an experimental investigation on the flow of a non-Newtonian fluid between rotating parallel disks. These results are qualitatively different from those exhibited by linearly viscous flows of the non-Newtonian fluid, where exceedingly high velocity gradients appear. Jimeneza and Kostic (8) developed, designed, and fabricated an innovative, Couette-type viscometer/rheometer with the main objective being to measure viscosity and elastic properties of low-viscous, non-Newtonian, and visco-elastic fluids, like dilute polymer solutions. Shackelton and Green (2) fabricated an on-line viscometer that has no moving mechanical parts. Measurements of the differential pressure developed along the measurement section and the temperature of the fluid are combined with the axial velocity of the test fluids to determine their mass/volume flow rates and viscosity values of both glucose syrup and melted chocolate. Chocolate exhibits non-Newtonian and glucose syrup exhibits Newtonian behavior.
However, the art is lacking an improved viscometer capable of measuring viscosity of both Newtonian and non-Newtonian fluids as described in the present disclosure. The present disclosure provides for a multiple capillary viscometer, a dual capillary viscometer, and a single capillary viscometer. In addition, the viscosity equations suitable for the described viscometers are provided and illustrated for the application to Newtonian fluids and non-Newtonian fluids. The test materials for the viscosity equation include 50% (by weight) sucrose aqueous solution as an exemplary Newtonian fluid and carboxymethylcellulose (CMC) aqueous solutions as exemplary non-Newtonian fluids.